Stencils depth-order is now saved on editing

When you are editing overlapping stencils, SpiderScribe will now remember their last depth-orders (the latest added stencils were always on top before). To change a stencil’s z-order simply click on it – it will bring it to the top. If you are in a view only mode, it will still bring selected stencils to the front, but will not save their new depth position.